This invention relates to a hydraulic ram particularly for use in the thrust-jacking of tunnel linings, for advancing a mine roof support unit, or for use as a mine prop.
A known type of hydraulic ram has a cylindrical outer member, and a cylindrical inner member reciprocable within the outer member. An extension member is reciprocable within the inner member, and a clamping ring is provided for locking the extension member to the inner member.
British Patent Specification No. 1,243,870 describes a mine prop of this type. In this case, the extension member is locked to the inner member, by means of the clamping ring, during the working stroke of the ram (that is to say when the inner member is extended). The piston face of the inner member which is loaded during the working stroke is the larger (circular) piston face, so that a large force is always available for the working stroke. The extension member is extended with respect to the inner member, in stages, by repeated retractions of the inner member. Each time the inner member is retracted, the clamping ring is released; and each time the inner member is subsequently extended, the clamping ring is engaged so that the extension member is extended with the inner member. When the extension member needs to be retracted, the inner ram must be repeatedly retracted and extended, whilst the clamping ring is alternately released and closed. Since the large volume of the working chamber associated with the piston of the inner member has to be repeatedly pressurised for this purpose, this procedure is relatively time-consuming. Moreover, the opening and closing of the clamping ring has to be done manually. This is also relatively time-consuming. Furthermore, owing to the weights of the clamping ring parts, considerable force needs to be applied to do this.
An object of the invention is to provide a hydraulic ram of this type whose extension member can be retracted rapidly and without the need to retract the inner member several times.
Another object of the invention is to provide an improved form of clamping ring for such a hydraulic ram, which ring can more rapidly and easily be opened and closed.